Silk screens are widely used by the printing industry throughout the world. They are constructed by stretching a fine weave material such as nylon or polyester around a wooden or metallic frame.
The silk screen is first coated with an emulsion of polyvinyl alcohol-polyvinyl acetate which also contains photosensitive catalysts such as diazo compounds, cupric dichromate, chromic nitrate or sodium bichromate. The image of the design to be printed is transferred on the silk screen by the following method. A negative of the image is prepared on regular photographic film and is placed on the coated and air-dried silk screen. The silk screen is then exposed to intense light to activate the catalysts. The emulsion on the areas of the silk screen not covered by the image becomes hard, impervious to water, solvents or ink and can no longer be removed with water. The areas of the emulsion covered by the image are protected from the intense light, do not harden and can be removed by a high pressure water jet stream. The result is a finished silk screen which is covered entirely by the hardened emulsion except for the area which was covered by the image.
In order to transfer the image from the silk screen to the desired substrate, ink is applied on one side of the silk screen. The screen is laid on the surface to be printed and the ink is smeared under pressure with a squeegee. The silk screen is then lifted leaving behind the image on the substrate.
Once a certain job is completed and the used silk screen no longer needed, it is desirable to reclaim the silk by removing completely the hardened stencil emulsion and the ink. The different chemical natures of the ink and the stencil emulsion make it extremely difficult to effectively remove both items with one chemical formulation. Inks, for example, can be removed with aromatic and aliphatic hydrocarbons, with chlorinated hydrocarbons and with ketones. These solvents, however, do not remove the hardened polyvinyl alcohol-polyvinyl acetate emulsion. On the other hand, chemicals such as alkali metal hydroxide solutions and bleach which soften the hardened stencil emulsion do not remove the ink. These chemicals are not compatible or soluble in the ink solvents and cannot be combined with them into one formulation.
Traditionally, silk screens are reclaimed by removing the ink with an organic solvent. The hardened emulsion is removed with a bleach or an aqueous caustic solution in a separate step. This is a time consuming and undesirable method.
The Fremont Industries (U.S. Pat. No. 3,853,782) silk screen cleaner utilizes an aqueous solution of alkalies and surfactants capable of softening the hardened stencil emulsion and the ink simultaneously. This process, however, has some very serious dissadvantages. They are as follows:
1. This solution must be heated and maintained at 180.degree.-250.degree. F. to be effective. This requires expensive heated tanks. Maintenance and energy costs for such a system can become undesirable to the user. PA1 2. The silk screen must be immersed for a minimum of fifteen minutes in the heated solution to obtain satisfactory results. PA1 3. This solution often leaves residual spots which must be removed with a strong solvent such as methyl ethyl ketone. PA1 4. This solution often leaves ghost images.
The present invention relates to a non-aqueous liquid chemical formulation which is capable of softening and removing simultaneously both the hardened emulsion and the ink at ambient temperatures. This formulation makes it possible to reclaim the silk screen in approximately five to seven minutes, depending on the method of contact.
These and other objects, features and advantages of the present invention will become apparent from a review of the preferred embodiment of the invention and the appended claims.